U.S. Pat. No. 6,526,951-B2, for example, discloses an electromagnetic combination valve provided with a first valve member (hereinafter referred to as first valve) and a second valve member (hereinafter referred to as second valve). The first valve opens when a solenoid coin in an electromagnetic driving portion attracts a moving core to one side in its axial direction. The second valve lifts off a cylindrical valve seat of a housing and opens when a backpressure, which acts on the second valve in a direction to close the second valve (in a valve-closing direction of the second valve), decreases to a valve-opening pressure of the second valve and a spring force of a coil spring urges the second valve to the one side in the axial direction. In the electromagnetic combination valve, the first valve forms a valve element of an electromagnetic opening/closing valve, and the second valve forms a valve element of a pressure-sensing valve. The pressure-sensing valve means a valve device with a valve-opening property to open when the backpressure, which acts on a rear face (pressure-receiving face) of the second valve, becomes smaller than the spring force of the coil spring.
In the electromagnetic combination valve, the second valve is provided with a rubber contact member (hereinafter referred to as sealing rubber), which is to come in contact with the valve seat of the housing, and the housing is formed from resinous material. Thus, in the case that the electromagnetic combination valve is used as a pressure-sensing valve in gasoline or gasoline vapor, adhesive substance may be deposited out of the resinous material and the rubber material. As a result, the adhesive substance may stick the sealing rubber of the second valve on the valve seat of the housing, to delay a valve-opening timing of the second valve or to lean the second valve. At worst, the second valve may be adhered in its valve-closing state and go out of order.
In this regard, it is considered to form a non-adherent surface treatment coating on the surface of the rubber formed part such as the sealing rubber of the second valve, to decrease the adherent degree or the frictional coefficient of the rubber formed part. As a method to decrease the adherent degree or the frictional coefficient of the surface of the rubber formed part, JP-H10-130428-A, for example, discloses a surface treatment method to form a fluorocarbon resin coating, a molybdenum disulfide coating or a diamond-like carbon (hereinafter referred to as DLC) coating on the surface of the rubber formed part.
In the case that the surface of the sealing rubber of the second valve has the fluorocarbon resin coating, the surface of the sealing rubber of the second valve is hardened under severely low-temperature condition, to decrease the airtight performance (to increase the fluid leakage degree) of the second valve, and the thickness of the coating must be large. Accordingly, the fluorocarbon resin coating is not suitable for the sealing rubber of the second valve that requires fine airtight performance under severely low-temperature condition. In the case that molybdenum disulfide powder is adopted as the material of the surface treatment coating put on the surface of the sealing rubber of the second valve, the surface of the coating has asperities due to the grain of the molybdenum disulfide powder. Accordingly, the molybdenum disulfide coating is not suitable for the sealing rubber of the second valve that requires fine airtight performance under severely low-temperature condition as the fluorocarbon resin coating is.
In the case that DLC is adopted as the material of the surface treatment coating of the sealing rubber of the second valve, DLC coating having a smooth surface and small thickness is effective to prevent the rubber gasket of the rubber formed part from sticking in contrast to the case that fluorocarbon resin coating or molybdenum disulfide coating is adopted as the material of the surface treatment coating. However, the hardness of DLC coating substance is around Hv 1000 and it is too large. As a result, it is difficult to secure airtight performance at the gap between the sealing rubber of the second valve and the valve seat of the housing in a valve-closing time of the second valve, that is, when the sealing rubber of the second valve is seated on the valve seat of the housing, under low-temperature condition.
Further, the DLC coating, which is adopted as the material to secure gas impermeability in plastic bottles formed from polyethylene terephthalate resin, has hardness around Hv 10, and it is too small. Thus, if the DLC coating is adopted as the material of the surface treatment coating of the sealing rubber of the second valve, the thin DLC coating on the surface of the sealing rubber of the second valve wears while the second valve repeats opening and closing operations, that is, the sealing rubber of the second valve repeats seating on and lifting off the valve seat of the housing. Accordingly, the durability and reliability of the DLC coating decreases, and it becomes impossible to keep the non-adhesive performance of the DLC coating.
It may also be considered to adopt a rubber material, which has fine flexibility and rich elastic deformability even under low-temperature condition, as the sealing rubber of the second valve. However, in the case that the sealing rubber, which has fine airtight performance under low-temperature condition, is adopted as the sealing rubber, if the second valve is kept in valve-closing state for a long period, the sealing rubber of the second valve is strongly stuck to the valve seat. If the sealing rubber is forcedly detached from the valve seat, the sealing rubber can be damaged, to decrease the durability and reliability of the airtight performance of the second valve under low-temperature condition.